1. Field of the Invention
This invention relates to a steel cord for the reinforcement of rubber articles such as rubber hose, conveyor belt, pneumatic tire and the like, and more particularly to a steel cord for the reinforcement of rubber articles having a two-layer construction made of a core consisting of two steel filaments and a sheath consisting of nine steel filaments arranged around the core as well as a pneumatic radial tire using the same in a radial carcass and/or a belt.
2. Description of Related Art
Steel cords have widely been used as a reinforcement for rubber articles such as rubber hose, conveyor belt, pneumatic tire and the like. For example, there is known a steel cord having a 3+9+15 construction formed by twisting steel filaments having the same filament diameter so as to change a twisting pitch every layer as a typical steel cord for reinforcing a heavy duty pneumatic tire. Such a steel cord is comprised of a core consisting of three steel filaments, an inner layer sheath arranged around the core and consisting of nine steel filaments, and an outer layer sheath arranged around the inner layer sheath and consisting of fifteen steel filaments.
In the steel cord having the above 3+9+15 construction, there is no gap of penetrating rubber into space portion inside the cord, so that if water contacts with the cord, it penetrates into the space portion of the cord not filled with rubber to corrode the inside of the cord and finally water propagates along the space portion in the longitudinal direction of the cord to undesirably enlarge the cord corrosion.
For this end, JP-U-64-30398 proposes a steel cord having two-layer or three-layer construction formed by arranging plural steel filaments in parallel to each other to form a core and twisting steel filaments at the outside of the core so as to create gaps between the steel filaments of the sheath layer. In this steel cord, rubber is easy to penetrate into the central portion of the cord through these gaps, so that the corrosion hardly occurs.
In the example described in JP-U-64-30398, however, the core is formed by arranging three or more steel filaments in parallel to each other, so that a space not penetrating rubber is formed in a center of the core. For instance, when such a steel cord is used in a belt of the tire, if a tread of the tire is subjected to cut damage in the running on bad road, water penetrates into a space portion not penetrating rubber inside the cord to cause corrosion in the inside of the cord. Further water propagates along the space portion in the longitudinal direction of the cord to enlarge the cord corrosion to finally cause separation failure.